This invention relates to a method and means for providing temperature compensation for measurements made with potentiometrically operated ion-selective (sensitive) field effect transistor (ISFET) probe assemblies operating at temperatures other than the calibration temperature.
For many years glass electrode probe assemblies which are selective to hydrogen ions have been used to measure pH. It is well known that these elecrode assemblies are temperature sensitive so that their output must be compensated for the temperature of th solution being measured. The following relationships describe the temperature sensitivity of such electrode assemblies: ##EQU1## where pH is the indicated or displayed value, Std. represents the standardization (calibration) value, R1n10/F is the Nernst factor, T is the absolute temperature, R is the universal gas constant, F is Faraday's constant and E is the glass electrode-reference electrode voltage. The number 7 represents the isopotential pH for the glass electrode system, which is that pH value at which the electrode output is independent of temperature. At that pH value the output for the glass electrode system is zero volts, which is the isopotential voltage.
Typically, the correction of these electrodes for their temperature sensitivity involves only correction for the glass-electrolyte interfaces by adjustment of the amplification of the instrument measuring the voltage E in accordance with the Nernst factor with the temperature component of that factor being measured by a separate sensor. The temperature sensitivity of other elements of these probes, such as the external reference electrode, is cancelled out due to the fact that there is an opposing internal reference electrode as part of the glass electrode assembly.
In 1970 P. Bergveld developed the ion-selective field effect transistor (ISFET) which can be used to measure pH in place of the glass electrode. Such a transducer is basically a metal oxide semiconductor field effect (MOSFET) device whose construction differs from the conventional MOSFET devices in that the gate metal is omitted and special techniques are employed to render the gate region selective to the ions of interest. An example of a method which can be used to construct a suitable ISFET is described in U.S. Pat. No. 4,505,799, issued to Ronald D. Baxter, a coworker of mine, on Mar. 19, 1985.
In Bergveld's article, THE OPERATION OF AN ISFET AS AN ELECTRONIC DEVICE, Sensors and Actuators, 1 (1981) 17-29, he discusses the temperature sensitivity of ISFETs. He points out that the temperature drift of ISFETs involves problems beyond those encountered with glass electrodes since the reference electrode voltage and the standard potential of the electrolyte-oxide interface are variable with temperature as are certain solid state parameters. He also states, "A usual approach in electronics to compensate for temperature drift in solid-state devices is to create a differential pair on one chip from which one device is the active input device and the other is used for temperature compensation, assuming the temperature characteristics of both devices are equal." As he further states, "Both requirements can be met reasonably for a pair of MOSFETs with today's MOSFET technology and the application of electronic feedback . . . . It is, however not realistic to use this approach for a pair consisting of an ISFET and A MOSFET on the same chip . . . . " He goes on to describe his suggested approach to temperature compensation as one involving a pair of ISFETs, one for measuring pH and one with a separate compartment on top of the gate, filled with a buffered agarose, which is in contact with the solution to be measured via a liquid junction. He then sets forth the problems with that type of arrangement and concludes as follows: "The conclusion is that the approach of a differential pair construction on one chip to prevent temperature drift, as commonly in use for MOSFETs, cannot be applied directly to ISFETs." He also concludes that " . . . the simultaneous measurement of the temperature with a separate sensor cannot be used for compensation of temperature drift in V.sub.g (T) for I.sub.d =constant. Instead of this, we have continuously to measure the unknown function for each individual ISFET connected to the amplifier during operation. With this measure, the set value of I.sub.d can be controlled in such a way that V.sub.g =constant. The same signal can be used to adjust the amplification of the measured output signal as a function of pH, in agreement with the slope correction of glass membrane electrodes." The author also states that " . . . the temperature compensation mentioned above . . . does not correct changes in the voltage of the reference electrode . . . and the electrolyte-oxide standard potential . . . as a function of the temperature." It is, of course, important to provide for compensation for the temperature sensitivity of the reference electrode in order to have complete temperature compensation for the ISFET measurement. Thus, Bergveld's comments indicate that he did not know how to accomplish a complete temperature compensation of the ISFET.
In a paper entitled A CHEMICAL-SENSITIVE INTEGRATED-CIRCUIT: THE OPERATIONAL TRANSDUCER, published in Sensors and Actuators, 7 (1985) 23-38, the author, A Sibbald, outlines the three principal attempts which had been made previously to negate thermal sensitivity, as follows:
(1) Operation at a fixed, athermal I.sub.d (a locus in the I.sub.d /V.sub.gs characteristic where the thermal effects are virtually self cancelling).
(2) A.C. signal injection technique which involves the injection of a high frequency signal into the ChemFET bulk with discrimination between the a.c. and d.c. components of the device output signal and thereby deriving separate signals related to chemical activity and to temperature.
(3) On-chip reference electrode. This electrode uses a pair of ChemFETs fabricated on the same chip such that the surface of one device is coated with a buffered 1% agarose gel and then encapsulated using epoxy, with a glass microcapillary forming a liquid-junction through the epoxy between the gel and the ambient thus providing a pH insensitive device and an adjacent pH sensitive device. A differential amplifier is then used for the measurement.
This author, in stating that he uses an array of ChemFETs operated at or near the athermal I.sub.d value, also indicated that it is nevertheless necessary to incorporate a miniature heat exchanger in the analysis system in order to minimize thermal effects and that it is essential that the threshold voltages of the individual ChemFet devices in the array are similar, which cannot be always guaranteed. He has thus indicated that no satisfactory, simple temperature compensation system had been devised when he wrote the paper.
It is amply evident from the above that temperature compensation of ISFETs, as practiced before the present invention, has either failed completely or required cumbersome systems, such as miniature heat exchangers. This has been so because ISFET assemblies exhibit three temperature sensitivities instead of one, as with the glass electrode. One is the Nernst temperature sensitivity, similar to that described above for the glass electrode. A second is the temperature sensitivity of the field effect transistor (FET) portion of the assembly. The third is the temperature sensitivity of the single reference electrode in the assembly.
In addition to the temperature sensitivity of the FET portion of the assembly, it has been found that current methods of semiconductor fabrication may lead to variations of the isopotential voltage of an ISFET assembly in excess of 20 mv. Such variations have been found to have the effect of limiting the accuracy of temperature compensation in systems where the ISFETs must be interchangeable. For example, it has been found that deviations of the isopotential voltage must be kept within a range of plus or minus 20 mv. in order to obtain an accuracy to 0.1 pH at any pH, over a temperature range of 0.degree.-100.degree. C.
It is an object of this invention to provide a method and means for compensating for the temperature sensitivity of all components of the ISFET assembly without using complicated circuits and without the need for complicated structure.
It is also an object of this invention to provide a method and means for overcoming the inaccuracies in temperature compensation in systems using interchangeable ISFETs as may be introduced by variations in semiconductor fabrication.